Detective Carl Morck has received a bottle that holds an old and decayed message written in blood. It's a cry for help from two young brothers, tied and bound in a boathouse by the sea. After floating in the ocean for years before turning up, the bottle sat forgotten, unopened, on a police department windowsill, before the seal was cracked and the gruesome message, written in Danish, was analyzed. Could it be real? Who are these boys, and why weren't they reported missing?